The present invention relates to power supply apparatus and methods of operation thereof, and more particular, to AC power supply apparatus and methods.
Power supply circuits are commonly used in equipment such as uninterrupted (or "uninterruptible") power supplies (UPSs), motor drives, and other applications. Conventional UPSs use a variety of different circuit topologies, including standby, line-interactive and on-line topologies. Generally, each of these topologies has advantages and disadvantages and, accordingly, selection of a particular topology is typically governed by the needs of the application.
For example, a typical standby UPS topology includes a transfer switch that directly connects the load to a primary AC power source under normal conditions and that transfers the load to a secondary AC source derived from a battery or other auxiliary source when the primary AC power source fails. Due to the time needed to operate the transfer switch, such a standby UPS often exhibit a significant interruption in power delivered to the load, which may be unacceptable for data processing or other critical tasks. In addition, standby UPSs often do not compensate for power quality, e.g., voltage degradation, harmonic distortion and low power factor. Nonetheless, the standby topology is often used for low-cost UPSs, as it is often cheaper to produce than other topologies. A line interactive UPS may use a transfer switch arrangement similar to that of the standby topology, but may also include means for regulating and conditioning the AC power source to improve power quality.
Many UPSs use an online topology. For example, a typical online series train UPS includes a series train AC/AC converter that produces an AC output voltage at a load from an AC input voltage provided by an AC power source such as a utility, but includes a DC link that is used to isolate the load from disturbance and other degradation of the AC power source. The intermediate DC bus is typically coupled to an inverter that inverts the DC voltage on the DC bus to produce an AC output. Other circuits, such as filters and regulators, may be included in the path with the rectifier and the inverter. Typically, the DC bus is also coupled to an auxiliary source of power, such as a battery, which maintains the DC voltage on the DC bus in the event the AC power source fails. Some online UPSs use circuit topologies other than a series train, including more complex topologies, such as delta converters, or other techniques.
Under normal operating conditions, online UPS's supply power to a load through a rectifier/inverter chain or similar regulating circuitry, providing relatively clean and regulated power at the output of the UPS. When the AC power source fails, the UPS may achieve an uninterrupted transition to battery power, as there typically is no need to change the state of a transfer switch. Some online series train UPSs also include a bypass feature such that, in the event of a failure, the inverter is prevented from supplying power to the load (e.g., by disconnecting it from the load or placing it into an inactive standby state) while the load is connected to the AC power source via a bypass path. Such a feature may also be used to provide an "economy" mode of operation, as power dissipation associated with the operation of the rectifier/inverter chain may be reduced when the load is transferred to the bypass path.
However, such an approach may be compromised if the AC power source is subject to small but frequent excursions outside of the power quality criteria, as may be the case, for example, in systems powered by an unsteady AC power source, Such as an auxiliary generator set. In such a case, the UPS may be forced to power the load through the inverter in order to meet the power quality requirements of the load, thus reducing the opportunity for greater efficiency through bypass operation.